Third generation (3G) wireless communication systems, such as the Universal Mobile Telecommunication System (UMTS), permit interoperability between radio access networks, such as the Global System for Mobile Communications (GSM) and Universal Terrestrial Radio Access Network (UTRAN). For this reason, UMTS user equipment (UE), such as mobile radiotelephones, can support multiple radio access technologies (RATs). For example, a UE may support communication using either Wideband Code Division Multiple Access (WCDMA) or GSM RATs, and thereby permit access to wireless communication services via network equipment associated with UTRAN or GSM cells.
For interoperability, the UE implements a process that controls selection and reselection of cells associated with both RAT types, i.e., inter-RAT cell reselection. As an illustration, when a mobile UE is in GSM idle mode, it may perform signal measurements of neighboring UTRAN cells. If the UE determines that one of the neighboring UTRAN cells has a stronger signal level than the serving and neighboring GSM cells, and the signal level satisfies a network-specified threshold, the UE selects the UTRAN cell for communication services.
Upon selection of the UTRAN cell, the UE switches to the UTRA communication mode and attempts to “camp on” the UTRAN cell. To successfully camp on the UTRAN cell, however, the cell must also satisfy one or more additional cell selection criteria. Exemplary cell selection criteria are specified in Third Generation Partnership Project (3GPP) Technical Specification 25.304. If the UTRAN cell does not satisfy the cell selection criteria, the UE deselects the UTRAN cell and switches back to GSM mode.